In recent years, accompanied by downsizing and enhanced performance of mobile devices, a non-aqueous electrolyte secondary battery has improved properties as a non-aqueous electrolyte storage element having high energy density and become widespread. Also, attempts are underway to improve gravimetric energy density of the non-aqueous electrolyte secondary cell, aiming to expand its application to electric vehicles.
Conventionally, a lithium ion secondary battery including a positive electrode of a lithium-cobalt composite oxide, a negative electrode of carbon, and a nonaqueous electrolyte obtained by dissolving lithium salt in a nonaqueous solvent has been widely used as the nonaqueous electrolyte secondary battery.
Meanwhile, there is a nonaqueous electrolyte secondary battery, which is charged and discharged by intercalation or deintercalation of anions in a nonaqueous electrolyte to a positive electrode of a material, such as an electroconductive polymer, and a carbonaceous material, and by intercalation or deintercalation of lithium ions in the nonaqueous electrolyte to a negative electrode of a carbonaceous material (this type of battery may be referred to as “dual carbon battery” hereinafter) (see PTL 1).
In this dual carbon battery, as indicated by the following reaction formula, the cell is charged by intercalation of anions such as PF6− and so on from the non-aqueous electrolyte to the positive electrode and by intercalation of Li+ from the non-aqueous electrolyte to the negative electrode, and the cell is discharged by deintercalation of anions such as PF6− and so on from the positive electrode and deintercalation of Li+ from the negative electrode to the non-aqueous electrolyte.PF6−+nC Cn(PF6)+e−  Positive electrode:Li++nC+e−LiCn  Negative electrode:                 charging reaction                    discharge reaction                        
A discharge capacity of the dual carbon cell is determined by an anion storage capacity of the positive electrode, an amount of possible anion release of the positive electrode, a cation storage amount of the negative electrode, an amount of possible cation release of the negative electrode, and an amount of anions and amount of cations in the nonaqueous electrolyte. Accordingly, in order to improve the discharge capacity of the dual carbon battery, it is necessary to increase not only a positive electrode active material and a negative electrode active material, but also an amount of the nonaqueous electrolyte containing lithium salt (see NPL 1).
As described above, a quantity of electricity the dual carbon battery has is proportional to a total amount of anions and cations in a nonaqueous electrolyte. Accordingly, the energy stored in the battery is proportional to a total mass of the nonaqueous electrolyte in addition to a positive electrode active material and a negative electrode active material. Therefore, it is difficult to enhance the weight energy density of the battery. When a nonaqueous electrolyte having a lithium salt density of about 1 mol/L, which is typically used in a lithium ion secondary battery, is used, a large amount of the nonaqueous electrolyte is necessary compared to a case of a lithium ion secondary battery. When a nonaqueous electrolyte having a high lithium salt density, i.e., about 3 mol/L, on the other hand, there is a problem that a reduction in a battery capacity is large as charging and discharging of the battery are repeated.
The operating voltage of the dual carbon battery is in the range of about 2.5 V to about 5.4 V, and the maximum voltage thereof is higher than that of a lithium ion secondary battery (about 4.2 V) by about 1 V. Therefore, the nonaqueous electrolyte tends to be decomposed. Once the nonaqueous electrolyte is decomposed, generation of gas, or excessive formation of a film of fluoride on a surface of an electrode is caused, which leads to reduction in the battery capacity or deterioration of the battery. Therefore, it is necessary to provide a countermeasure for fluoride generated by decomposition of the nonaqueous electrolyte.
In a nonaqueous electrolyte secondary battery as a nonaqueous electrolytic capacitor element, moreover, a non-electroconductive film, so called a solid electrolyte interface (SEI) is formed at the time of an initial charging and discharging. The SEI prevents decomposition and deterioration of a negative electrode, which may be caused by a strong reduction reaction at the time of charging, and generation of gas due to decomposition of the nonaqueous electrolyte. As the electrolyte salt density is increased to increase the discharge capacity, however, a SEI is not desirably formed. Therefore, there is a problem that the charge capacity is lowered as a number of a cycle of charging and discharging is increased.
Moreover, disclosed is an example using a nonaqueous electrolyte containing a compound having a site capable of bonding to anions (see NPL 2 and NPL 3).
However, in these disclosed techniques, a positive electrode containing a positive electrode active material capable of intercalation and/or deintercalation of anions is not discussed. Moreover, NPL 2 discusses use thereof only to the charge voltage of about 3.8 V, and NPL 3 discusses use thereof only to the charge voltage of about 4.1 V, and both of them have not discussed use thereof at high voltage. Moreover, both literatures discuss use of the aforementioned nonaqueous electrolyte with the electrolyte concentration of about 1 M, not with a high concentration thereof.